1. Field of the Invention
The invention relates generally to the field of acoustic environment simulations, and more particularly to digital devices for providing an accurate simulation of the ambience created by sound reflections in auditoriums.
2. Description of the Prior Art
A relatively recent concern in audio recording and reproduction has developed regarding the effect on the listener of sound which is reflected from the surfaces within a concert hall or auditorium. Such reflections affect not only direction of perceived sound, but also the time that it takes for the sound to reach the listener. A large auditorium will produce reflected sounds which arrive at the listener after delays of substantially greater duration than anything that would naturally occur in a person's living room. An exact reproduction of the originally created sound in a small living room would sound considerably different than if that same sound were produced in a large concert hall. In fact, the same originally created sound produced in different auditoriums would sound different. Several approaches have been taken to provide for the production in the home of the effect of various concert halls.
Stereo itself represents a step toward providing realism to the home production of sounds, but cannot present a range of sound directions large enough to encompass the reflected sounds. Binaural recording does provide the reflected sounds, but is generally limited to playback by headphones. A quadraphonic sound system can provide a good representation of the effect of an auditorium if it has truly independent channels. Both binaural and quadraphonic systems require that the music be specially recorded to produce the effect desired. Hence, present stero systems and recordings not incorporating those practices are incompatible.
An alternative approach is to electronically simulate the effect of the concert hall reflections at the time the recorded sound is reproduced. This requires a simulator at the site of such reproduction of the sound, that is a simulator in the home. One significant advantage of this approach is that by building the simulator with some range of control over what it is simulating, a single simulator can be used to simulate more than one type of auditorium, and the choice is within the control of the home listener.
While the terminology used to refer to the delayed sounds caused by reflections is not yet universal, reverberation will be used herein to refer to all of such sounds, ambience will be used to refer to the early reflections (those reaching the listener after a short delay) and reverberance will be used to refer to the later delayed sounds. The distinction between ambience and reverberance is a matter of perception, and an elapsed time which divides the two can be anywhere from 50 ms to 150 ms depending on the size and shape of the auditorium. The distinction is based on the fact that the listener is able to perceive individual components of the sparser early sounds, whereas the density of reflections of the later reverberation is so great that they merge together in the perception of the listener. The larger the room, the longer the delay for the first reflections, and the greater the period of ambience.
One system of simulating the various delayed sounds which results from reflected sound is the invention of Cohen, U.S. Pat. No. 4,283,600, which describes a series of electronic delay lines with a speaker associated with the output of each delay line such that the total audio output is the composite of the speakers, all producing essentially the same signal, but at different amounts of time delay, thus providing a simulated ambience. The need for multiple speakers, and a power amplifier associated with each such speaker, may be unacceptable for some applications.
The invention of Morgan, U.S. Pat. No. 4,338,581, discloses a system wherein an analog signal is delayed through employment of various analog delay devices, and digital techniques are used for selecting the specific time delays and corresponding amplitudes for producing the resulting output signal. Morgan provides means by which the actual combination of selected delays and amplitudes can be programmed into the simulator. This system however has a very limited number of finite delays, and adds to those delays additional signals produced from a comb filter with exponential decay of the signal amplitudes. While this will produce a form of reverberation which may be pleasing, it does not allow the realistic simulation of the delays, amplitudes and directionality of the sounds of the entire period of ambience in normal auditoriums.
In U.S. Pat. No. 4,105,864, Berkovitz discloses a simulator which includes digitization of the sound, and storage in a digital memory. That invention uses multiple speakers as does Cohen, however, and it is a complicated process to change the amplitude and delay parameters.